1. Field of the Invention
The present invention relates to fuel injection control means for internal combustion engines, and more particularly to fuel injection control means for engines having heavy duty fuel injection valve means and light duty fuel injection valve means, the operation of which are controlled in accordance with the loads on the engines.
2. Description of the Prior Art
Conventionally, it has been known in an internal combustion engine to provide a primary intake passage and a secondary intake passage which are substantially independent from each other but leading to the same combustion chamber. The secondary intake passage is usually provided with a shutoff valve which is closed under light load engine operation so that the intake air is drawn substantially through the primary intake passage for light load operation. It has also been proposed in this type of intake system to provide a primary fuel injection valve and a secondary fuel injection valve, respectively, in the primary and secondary intake passages, and to operate primary fuel injection valve throughout the engine operating range but to operate the secondary fuel injection valve under the heavy load engine operation wherein the secondary intake passage is opened. This type of fuel injection control is shown for example in Japanese patent publication No. 53-43616 published on Nov. 21, 1978.
In this type of fuel injection control, it has been experienced that a stepwise change in the engine output torque is produced when the operation of the secondary fuel injection valve is started or stopped, even though the quantity of fuel supply is precisely calculated in accordance with the engine operating condition. This stepwise change in the engine output torque is caused by the fact that when the fuel injection from the secondary fuel injection valve is stopped, the liquid film flow on the wall surface of the secondary intake passage is evaporated and drawn into the combustion chamber making the air-fuel mixture temporarily rich. When the fuel supply through the secondary fuel injection valve is started, a part of the fuel is consumed to make the wall surface of the secondary intake passage wet so that the airfuel mixture becomes lean.
In order to eliminate the above problems, the quantity of fuel supply may be temporarily increased when the fuel supply through the secondary fuel injection valve is started. Further, the quantity of the fuel supply may be decreased when the operation of the secondary fuel injection valve is stopped. It has been found, however, that the simple increase or decrease of the quantity of fuel supply is not sufficient to eliminate the stepwise change in the engine output torque.